(i) Technical Field
The present invention relates to an image encoding apparatus and method, an image decoding apparatus, and a non-transitory computer readable medium.
(ii) Related Art
Hitherto, various systems for encoding or compressing images have been proposed. Such systems include Joint Photographic Experts Group (JPEG) and Block Truncation Coding (BTC). The former is a variable-length encoding system, which reduces the encoding amount while keeping image quality deterioration at a low level by combining frequency conversion and quantization. The latter is a fixed-length encoding system, which is capable of encoding an image to a certain fixed encoding amount by representing a block with information of a fixed length.
In the JPEG system, the upper limit of the encoding amount is not controlled, and the size of an apparatus tends to increase when multiple lines (such as eight lines) of an image are processed at the same time. Also, the BTC system produces a fixed encoding amount even in response to an input with a small information amount.
In some network environments, it may be assumable that, after a compressed image is sent, the image quality thereof is desired to be changed later. For example, it is desirable in a narrowband network environment to have a function of firstly sending a rough image and then sending the same image with an improved image quality again. This function demands securing of the upper limit without having an overhead of the amount of encoding, and compression in which the image quality is, for example, reversibly changeable.